In oil deep wells or in oil wells in severe corrosive environments involving corrosive substances such as humid carbon dioxide gas (CO2), hydrogen sulfide (H2S) or chloride ion (Cl−), austenite-ferrite two-phase stainless steel pipes having a large content of Cr such as 22Cr steel or 25Cr steel are used as oil well pipes.
These austenite-ferrite two-phase stainless steels as having been subjected to the solution treatment usually applied in the production thereof can attain at most such a tensile strength (TS) that a tensile strength is of the grade of 80 kgf/mm2 (785 MPa) and a yield strength (0.2% yield stress) is of the grade of 60 kgf/mm2 (588 MPa). In consideration of this issue, JP2-290920A discloses a method for obtaining a high-strength two-phase stainless steel pipe, wherein a two-phase stainless steel pipe that contains 0.1 to 0.3% of N is subjected to a cold rolling with a reduction of area of 5 to 50%, and thereafter the pipe is heated at 100 to 350° C. for 30 minutes or more to yield the intended pipe. In this case, it is mentioned that a high-strength two-phase stainless steel pipe is obtained by combining the work hardening due to the cold rolling with the aging treatment.
However, in these years, oil wells have a remarkable tendency toward being deeper, and hence, for the purpose of the use in environments more severe than those hitherto experienced, it is required to produce two-phase stainless steel pipes which are high in strength, in particular, of the grade of 110 to 140 ksi (the lowest yield strength is 757.3 to 963.8 MPa) and additionally have various strength levels defined in the specifications. Thus, for that purpose, it is necessary to consider not only the content of N but the contents of the other composition elements, and also it is necessary to more strictly control the extent of the cold rolling. The production method disclosed in JP2-290920A offers a problem of the production efficiency deterioration or the cost increase due to the addition of the aging treatment step.
For the purpose of attaining a high corrosion resistance and a high strength, JP7-207337A discloses a method in which a Cu-containing two-phase stainless steel material is subjected to a cold rolling with a reduction of area of 35% or more, thereafter heated and quenched, and then subjected to a warm working. In this document, presented is a conventional example disclosing data showing that a Cu-containing two-phase stainless steel wire rod is subjected to a solid-solution heat treatment and thereafter subjected to a cold rolling with a reduction of area of 25 to 70%, and thus a high-strength wire rod having a tensile strength of 110 to 140 kgf/mm2 has been obtained. However, in this case, disclosed is only the increase of the tensile strength due to the cold rolling, and the disclosed data is associated with a wire rod but not with a pipe, and hence it is not clear what is the level of the yield strength significant in the material design of the oil well pipes.
Further, JP5-277611A describes a high strength that can be attained by a low-working-ratio cold rolling based on forging. However, here is merely disclosed a method for improving the strength by successively forging with a cold rolling ratio of about 0.5 to 1.6% over the whole region, in the longitudinal direction, of a two-phase stainless steel stock that has been subjected to a solution treatment while the stock is being imparted with rotation.